The present invention relates to a device for controlling an AC motor which is used for driving a vehicle.
The AC induction motors are attracting attention as a promising substitute of a DC motor of an electric motor driven wheeled vehicle although the DC motors have been widely used due mainly to the fact that they are easy to control. As compared to the DC motors, they are simple in construction, easy to handle, of maintenance free because no brushes that are subject to wear are used, compact, light in weight, and suited for mass production. Besides after one charging operation, the motor vehicle with an AC induction motor can travel longer than with a DC motor. It is also possible to cause the AC motor to produce a drive torque as large as that required by the motor vehicle if appropriately controlled.
According to a known device for controlling an AC induction motor, if it is desired to change the direction of rotation to the reverse during forward running with the AC motor rotating in forward direction, a driver should step on a brake pedal to cause the AC motor to start regenerating braking to give a braking torque that is variable in proportion to the amount of brake pedal depression, and after the vehicle has come to a halt, he shifts a manipulator to the reverse to switch the direction of rotation of the motor by switching two motor drive signals among three-phase alternating drive signals.
The above mentioned copending U.S. application Ser. No. 810,686 discloses a device for controlling an AC induction motor. According to this proposed device, if a forward-reverse changeover switch is manipulated and the reverse is selected while the vehicle is travelling forward, the AC motor is caused to start regenerative braking. When the motor vehicle has come to a halt due to this regenerative braking, the two phases of the three phase motor drive signals are interchanged and the AC motor is caused to start power running. Since the regenerative braking of the motor is initiated upon recognizing a shift in position of the changeover switch, the processing of recognizing the shift in position of the change-over switch is needed. This processing requires a step of comparing the stored data for the position of the change-over switch with previously stored data for the position of the change-over switch.